The present invention relates to a cooled screw vacuum pump comprising two rotating systems, consisting each of a screw rotor and a shaft with a floating device supporting the rotors, having, on each shaft, two mutually spaced bearings and an empty space arranged in each rotor, open on the bearing side, wherein is located an element cooling the rotor internally.
In an already proposed screw vacuum pump of this type, the bearing of the floating support on the rotor side is located within a central hollow space, open toward the bearing side, inside the rotor. Cooling is effected with the aid of a lubricating oil, which is first passed, inside a central channel in the shaft, to the bearing on the side of the rotor. In known fashion, the transported oil volume is larger than is needed for lubrication of the bearing in order to be able to carry away the maximum amount of heat possible.
With respect to the screw vacuum pump, the oil volume which, according to the state of the art, can be passed through the empty space, is limited since it is not only the bearing but also the bearing support that must be accommodated in said empty space. Therefore, there is the risk of inadequate cooling on the pressure-side region of the screw vacuum pump since it is precisely in this region that the generated heat is greatest due to the executed compression work.
Because of the existing empty space inside the rotor, the wall thickness of the rotor is also limited in the bearing region of the empty space. As a result, it is only possible at very high temperature gradients, to carry off the heat developing in the pressure-side region of the screw threads via the suction side region of the rotor, the shaft and the cooling oil. High temperature or inadequate cooling of the pressure-side region of a screw vacuum pump results in uneven rotor expansions and thus in local clearance consumption between the rotors and between each of the rotors and the housing. Run-up of rotors may, in fact, be prevented by relatively large clearances.
Relatively large clearances, however, result in deterioration of the pump operating properties. Furthermore, with respect to the prior known screw vacuum pump, there exists the danger of overheating the bearing located in the empty space, all the more so since said bearing can only be lubricated with relatively warm oil. Finally, the prior known screw vacuum pump can only be operated with vertically arranged shafts.
The present invention is based on the object of equipping a screw vacuum pump of the initially mentioned kind with improved cooling means.
According to the invention, this object is solved by making use of the fact that the bearing on the rotor side of the support is located outside the empty space in the rotor. The invention facilitates effective cooling of the rotor from the inside without being impeded by the bearing and bearing support, so that the unwelcome clearance consumption will no longer occur in this critical region.
Each rotor appropriately consists of two segments with different thread profiles, whereby the thread depth of the pressure-side segment is smaller than the thread depth of the suction side segment. A lesser thread depth in the pressure-side segment provides more space for accommodation of the empty space needed for the internal cooling.
If, in addition, the rotor and housing are stepped in such manner that the pressure-side rotor segment has a smaller diameter than the suction-side rotor segment, then this measure creates more space in the housing for the accommodation of jacket cooling.
According to another characteristic of the invention, it is appropriate to additionally provide in the wall of the pump housing, i.e at least at rotor level, channels perfused by a cooling agent.
A cooling agent of this type permits, specifically in combination with the interior cooling of the rotor according to the invention, uniform tempering of the entire pump. Consequently, the pump is able to adopt variable temperatures with variable loads, without resulting in gap reductions. It is appropriate to also include in such tempering the bearings, the bearing supports and the driving motor, in order to prevent problems due to variable temperature expansions. Lastly, a jacket cooling of the proposed type has the benefit of having the effect of excellent sound deadening.